In recent years, a PTC element is used to prevent a secondary battery from overheating which is housed in an electronic/electric instrument such as a cellular phone (see, for example, Patent Literature 1).
As an example, a block diagram of a conventional electric circuit incorporating such PTC element for overheat protection is shown in FIG. 4. As in the drawings, a secondary battery 61 housed in an electronic/electric instrument (not shown) is electrically connected to an application circuit 63 via terminals 67a and 67b in the conventional electric circuit 60. Further, a PTC element 65 is inserted between the secondary battery 61 and the application circuit in series. Such PTC element 65 is located-on and intimately attached to the secondary battery 61 in the electronic/electric instrument to detect a temperature anomaly of the secondary battery 61.
When an application circuit 63 is operated by using the secondary battery 61 as an electric source in the electric circuit 60 as above, a temperature of the secondary battery 61 may increase by heat generated by a discharge reaction in the secondary battery 61. As the temperature of the secondary battery 61 increases, a temperature of the PTC element 65 also increases. Under a normal condition, the PTC element 65 is at a temperature less than its trip temperature and in a Low state. In this case, a resistance of the PTC element 65 is sufficiently low to exert substantially no influence on the discharge of the secondary battery 61.
However, it may be assumed that the discharge reaction in the secondary battery 61 excessively proceeds to cause a temperature anomaly (abnormal heating). In this case, when the temperature of the PTC element 65 exceeds the trip temperature under thermal influence of the secondary battery 61, the PTC element 65 trips from the Low state to the High state to steeply increase its electric resistance. Consequently, a current flowing through the PTC element 65, that is, a current flowing through the secondary battery 61 is significantly restricted. In this way the PTC element 65 is able to prevent the secondary battery 61 from overheating by detecting the temperature anomaly of the secondary battery 61. When a cause of the temperature anomaly has been eliminated, the PTC element 65 returns to the Low state again in due course.
In addition to the overheat protection of the secondary battery 61 as described above, the PTC element 65 has also a function of overcurrent protection.
Under the normal current condition with no overcurrent, The PTC element 65 is in the Low state and does not exert substantially no influence on the discharge of the secondary battery 61. However, if the current flowing through the electric circuit 60 becomes too large, Joule heat is considerably generated in the PTC element 65. When the temperature of the PTC element 65 exceeds its trip temperature, the PTC element 65 trips from the Low state to the High state to steeply increase in its electric resistance, and therefore the current flowing through the secondary battery is significantly restricted. In this way the PTC element 65 is also able to prevent the overcurrent flowing through the secondary battery 61 by detecting the current anomaly.
As known in this technical field, the PTC element show a reversible temperature-dependent electric characteristics (change in resistance), so that it can be used repeatedly (or it is resettable). Therefore, in a case of using the PTC element for preventing the secondary battery from overheating and so on, there is advantageously no need to replace it with new one as it would be required in a case of a fuse.    Patent Literature 1: JP-A-2001-102039    Patent Literature 2: JP-A-2002-528874